1. Field of the Invention
This invention relates generally to devices for quantitatively measuring the concentrations of constituents of multicomponent gaseous samples, and more particularly to devices that conduct such measurements based upon the mobility of ions in drift region.
2. Description of the Prior Art
Ion mobility spectroscopy is a powerful and well known method for providing quantitative measurements of the composition of a gaseous environment. In this context, the ion mobility sensor is usually designed for operation at atmospheric pressure although in principle it can operate at higher or lower pressures. The ion mobility sensor typically is comprised of two identifiable regions: (i) an ionization region where ions are formed representative of the gas sample to be analyzed, and (ii) a drift region into which the ions are injected and allowed to drift in an electric field before collection.
During the ions, drift in the drift region, the ions make many collisions with the sample gas and their motion in the electric field is characterized by the ion mobility. Since the ion mobility is a function of ion mass, ions of different mass segregate during their drift. Thus, by measuring the ion arrival time spectrum at the collector, a signature of the gas sample composition is obtained. Although positive ion spectra provide a selective signature for all gaseous species, the additional monitoring of negative ion spectra (for electronegative species) can provide supplementary information for either diagnostic or calibration purposes.
In currently available commercial instruments, the dimensions of the ionization region are comparable in magnitude to those of the drift region. The ions are produced primarily through ion-molecule reactions, where the impurity molecules of interest are ionized as a result of collisions with primary ions not necessarily derived from the background sample gas but rather from carrier gases deliberately introduced into the reaction region. These primary ions are produced as a result of collisions of beta particles emitted from a Ni-63 radioactive source (located within the ionization region) interacting with the sample and/or carrier gas. Since the beta particles are produced continuously, the ion production process is continuous and the species present in the ionization region represent an equilibrium composition developed over a relatively long time and which includes (deliberately in the case of the use of a carrier gas) ions produced as a result of ion molecule chemistry. Thus, in order to analyze the ions present in this equilibrium ion "sea", provision must be made to extract a pulse of ions into the drift region. This is usually accomplished using an electronic shutter whereby ions are only allowed into the drift region during the application of suitable electric potentials to the shutter electrodes. The detection limit of commercially available sensors depends on the particular species, and levels below one part per billion have been detected for certain organic molecules in air samples at atmospheric pressure without any preconcentration.
Although the ion mobility spectrometer is a well proven instrument, presently available models are relatively large, expensive and require ancillary gas supplies for calibration and for enhancing the ionization efficiency. In addition, present ion mobility spectrometers utilize a continuous nonadjustable ionization energy source. The ionization energy source is typically radioactive which is frequently undesirable. Because a continuous source has been presently used, a shutter is needed to inject a pulse of the total ions into the drift region. The shutter is typically a grid electrode that has a shutter voltage applied to it that injects the ions into the drift region.
The continuous source of the ionization energy is fixed and cannot be changed. Thus, when a complex molecule is being ionized, more than one species of ion may be generated if it interacts with sufficiently energetic electrons. This will disassociate the molecule into smaller fragments and some of those smaller fragments will also be ionized and so on. Therefore, there is very little control over which species of ion can be generated in current ionization methods.
In ion mobility spectrometers, the selectivity of the sensor is derived from the dependence of the ion mobility on ion mass. Thus, the transit time t.sub.k of ions of a particular species k is given by t.sub.k =d.sub.d /w.sub.k, where d.sub.d is the ion drift distance and w.sub.k is the ion drift velocity. The drift velocity is a function of E.sub.d /N, the ratio of the drift electric field to total gas density. Since the density of a noncompressible gas is related to the pressure p and absolute temperature T through the relation N=P/(KT), where K is Boltzmann's constant, it follows that changes in sample pressure, temperature, or both result in changes in the gas density N. If the field E.sub.d is fixed, then changes in N also lead to changes in E.sub.d /N and thereby w.sub.k. Thus, the expected arrival time t.sub.k of ions of species k changes with pressure or temperature. In order to provide calibrated time signatures at the collector, the sample pressure and temperature are usually monitored using separate sensors so that appropriate compensation can be applied to the collected raw data. The necessity for two additional sensors increases the hardware complexity and introduces further potential reliability concerns.
It would also be advantageous to provide ion mobility sensors with a means for triggering an alarm when the concentration of a monitored species rises above some preset value. Alternatively, it would be advantageous to provide a means for the measuring of one or more different species.